1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device which uses a switching device to control pixels.
2. Description of the Related Art
A liquid crystal display device generally widely available is in TN liquid crystal mode. A TN liquid crystal display device has a disadvantage that its viewing angle property is not better than that of a CRT. Liquid crystal display devices in IPS (In-Plane Switching) mode and MVA (Multi-domain Vertical Alignment) mode having an improved viewing angle property are developed, and are widely used as a display unit for electronic devices. In these devices, in an MVA liquid crystal display device, a protrusion in a bank shape or a recess in a slit shape is disposed in a pixel on at least one of a TFT substrate on which a switching device (TFT) is formed to control a liquid crystal pixel or a common substrate on which a color filter is formed. The protrusion or the recess controls the alignment of liquid crystal molecules. In the MVA liquid crystal display device, the alignment is split (multi-domain) by utilizing its property that the tilt directions of the liquid crystal molecules are varied on both sides of the protrusion (or the recess), and thus a wide viewing angle property can be implemented.
In the MVA liquid crystal display device before, a phenomenon occurs in which a screen looks whitish when it is viewed from an oblique direction. FIG. 15 shows a T-V (transmittance to voltage) property when a screen is viewed from the front side (a curve connecting solid diamonds), and a T-V property when the screen is viewed from the direction at an angle of 60 degrees upward with respect to the front side (a curve connecting asterisks) where the horizontal axis is the applied voltage (V), and the vertical axis is the transmittance. As shown in the area circled in FIG. 15, the transmittance viewed from the oblique direction is higher than the transmittance viewed from the front side when voltage slightly higher than a threshold voltage is applied to a pixel electrode. In addition, when the applied voltage becomes higher to some extent, the transmittance viewed from the oblique direction is lower than the transmittance viewed from the front side. On this account, when the screen is viewed from the oblique direction, the brightness difference in a red pixel, a green pixel and a blue pixel becomes small. Consequently, as described above the phenomenon occurs that the screen looks whitish. This phenomenon is called discolor. Discolor occurs in the MVA liquid crystal display device as well as in the TN liquid crystal display device.
For a scheme to solve this phenomenon, there is a method in which a pixel electrode is split into two areas, a switching device is connected to the pixel electrode in one of the areas to directly apply data voltage (data signal), and data voltage is applied to the pixel electrode in the other area through capacitance. In the pixel area to which data voltage is applied through capacitance, the capacitance ratio to the liquid crystal capacitance determines the voltage to be applied to liquid crystals, and this voltage is different from the voltage to be applied to the pixel area to which data voltage is directly applied. Thus, the phenomenon shown in FIG. 15 is suppressed that the transmittance viewed from the oblique direction is higher than the transmittance viewed from the front side, and consequently, the phenomenon is also suppressed that the screen looks whitish (discolor). As described above, the method in which a single pixel is split into a plurality of areas and the pixel areas capacitively coupled are used to improve the display property is called a half tone gray scale method by capacitive coupling (capacitive coupling HT method).
FIGS. 16A and 16B show the configuration of a single pixel of a TFT substrate of a conventional MVA liquid crystal display device before using the capacitive coupling HT method. FIG. 16A shows a plan layout depicting a single pixel. FIG. 16B shows a cross section sectioned at a phantom line A-A′ shown in FIG. 16A. Although not shown in the drawing, the MVA liquid crystal display device actually has the structure in which vertical alignment liquid crystals are sealed and sandwiched between a TFT substrate 101 and a common substrate (not shown) on which a color filter and other elements are laminated. For the sake of easy understanding, protrusions 61, 62 and 63 in a bank shape are additionally shown in FIG. 16A. The protrusions are formed on the common substrate side to control the alignment of liquid crystal molecules. In the liquid crystal display device, a multi-domain is formed of the protrusions 61, 62 and 63 and a recess that is formed of a space having a slope at an angle of 45 degrees between pixel electrodes 51 and 52 on the TFT substrate 101 side.
As shown in FIGS. 16A and 16B, on a glass substrate 100, a gate bus line 10 and an auxiliary capacitance bus line 11 are arranged in parallel with each other, which are formed of Ti, for example. An insulating film 20 formed of SiN is formed over throughout the front surface thereon. The gate bus line 10 is partially used as a gate electrode of a TFT 5 which switches a liquid crystal pixel. In the area of the TFT 5, an operative semiconductor layer 21 is formed which is made of amorphous silicon, and an impurity layer 22 is formed which is made of amorphous silicon doped with phosphorus. The insulating film 20 on the gate electrode functions as a gate insulating film. The following are formed of materials such as Al: a data bus line 31 which is orthogonal to the gate bus line 10 through the insulating film 20, a drain electrode 32 which is connected to the data bus line 31, a source electrode 33 which is disposed on the operative semiconductor layer 21 and faces the drain electrode 32 at a predetermined space, and control electrodes 35a, 35b and 35c which are connected to the source electrode 33. A protective insulating film 40 which is made of SiN is formed over throughout the front surface thereof.
A contact hole 41 is opened in the protective insulating film 40, and the control electrode 35a is electrically connected to the pixel electrode 51 in a first area and formed thereabove through the contact hole 41. In addition, the protective insulating film 40 is opened in terminal lead parts of the gate bus line 10, the auxiliary capacitance bus line 11 and the data bus line 31 outside a display area, not shown. In the gate bus line 10 and the auxiliary capacitance bus line 11, the insulating film 20 is also opened. The pixel electrodes 51 and 52 are formed of a transparent electrode such as ITO. The pixel electrode 52 is in a second area and separated from the pixel electrode 51 in the first area. The pixel electrode 52 is capacitively coupled to the control electrodes 35a, 35b and 35c through the protective insulating film 40. Thus, the capacitive coupling HT method is implemented, and discolor can be suppressed in the liquid crystal display device when viewed from the oblique direction. In addition, auxiliary capacitance is formed by the auxiliary capacitance bus line 11, the pixel electrodes 51 and 52 as well as the insulating film 20 and the protective insulating film 40 which are sandwiched therebetween.
Patent Reference: JP-Application-No. 2004-106138
In the capacitive coupling HT method, the control electrodes 35b and 35c are particularly required which are capacitively coupled to the pixel electrode 52 in the second area. The control electrodes 35b and 35c are formed at the same time when the data bus line 31, the drain electrode 32 and the source electrode 33 are fabricated in order not to increase the number of fabrication process steps, and they need an area to some extent to provide required control capacitance. The data bus line 31 is demanded to have a low resistance value to some extent because of a problem of a time constant when data voltage is written to a pixel, and thus metal such as Al is used for the data bus line 31. Therefore, for a transmissive liquid crystal display device, the control electrodes 35b and 35c overlapped with the pixel electrode 52 in the second area are the areas that light is not transmitted. On this account, a problem arises that the aperture ratio of the pixel is reduced and the brightness of a display device is dropped.
The liquid crystal display device needs auxiliary capacitance regardless of the use of the capacitive coupling HT method. Generally, the liquid crystals are driven at alternate current. However, a shift occurs in the effective voltage that is applied to liquid crystals depending on the cases in which a pixel is written at a positive voltage and it is written at a negative voltage. In order to relax the shift in the effective voltage and to prevent a phenomenon of image sticking or flickering in a display screen, auxiliary capacitance is provided in each pixel in the magnitude that is one to several times the liquid crystal capacitance. As shown in FIGS. 16A and 16B, auxiliary capacitance is formed at the area where the auxiliary capacitance bus line 11, the pixel electrodes 51 and 52 are overlapped with each other. In order to obtain a predetermined capacitance value, the area of the overlapping part is needed to some extent, and it is required to thicken the auxiliary capacitance bus line 11 and other schemes. Consequently, the auxiliary capacitance bus line 11 formed of metal such as Ti causes a reduction in the aperture ratio of the pixel.